Most three-bladed and two-bladed turbines are equipped with a yaw system to be able to change the orientation of the rotor axis in order to follow the wind direction and ensure that the rotor faces into the wind, thereby maximizing power production. Most three-bladed and two-bladed turbines modulate the pitch of the rotor blades to control the speed of the turbine and hence the power output of the turbine, as well as for shutdown. In most instances, wind turbines work at partial or whole variable rotor speed between the cut-in and the rated wind speed, while after the rated wind speed, the controller keeps the power and the running speed constant.
In two-bladed wind turbines, it is possible to yaw the turbine into or out of the wind also to control the speed of the turbine, and hence the power output of the turbine, without any need of pitch control. This is possible where the two-bladed turbine uses a teetering hinge (or “see-saw” hinge) to attach the turbine blades to the drive train of the turbine. The teetering hinge gives the rotor an additional degree of freedom, allowing the turbine to overcome gyroscopic forces, and to modulate the yaw angle sufficiently quickly to control the rotor speed of the turbine. By controlling the rotor speed of the turbine other than the rotor torque, it is possible to optimize power also when the wind speed is greater than the rated wind speed (i.e., the wind speed at which the turbine produces the max possible power when the turbine is facing into the wind (at zero yaw angle)).
At lower than rated wind speed, yaw control is used to keep the turbine facing into the wind, while the shaft torque is modulated to get the rotor work at its max aerodynamic efficiency irrespectively of the variations of the wind speed. In other words, when the wind speed is lower than the rated speed, the turbine is kept facing into the wind and the power, which is produced at highest efficiency, rises and falls with wind speed.